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Diagnostic Scope Testing

Key Benefits of a Tune-Up


Any tune-up today should start with a battery of performance checks to base line, or to
confirm the engine's overall condition.

These tests should include the following:

The Oxygen Sensor

Probably the most overlooked component of today’s engines!

Though some motorists don’t even know what an oxygen sensor is, let alone the engine may have one or more of these services, the fact remains that sluggish 02 sensors cause a lot of driveability problems. A recent EPA study found that 70% of all vehicles that fail an I/M 240 emissions test need a new sensor. An Oxygen sensor is a voltage generator. It is constantly making a comparison between the Oxygen inside the exhaust manifold and air outside the engine. If this comparison shows little or no Oxygen in the exhaust manifold, a voltage is generated.

The output of the sensor is usually between 0 and 1.1 volts. All spark combustion engines need the proper air fuel ratio to operate correctly. For gasoline, this is 14.7 parts of air to one part of fuel. When the engine has more fuel than needed (this is called running rich), all available Oxygen is consumed in the cylinder and gases leaving through the exhaust contain almost no Oxygen. This sends out a voltage greater than 0.45 volts. If the engine has less fuel than needed (this is called running lean) all fuel is burned, and the extra Oxygen leaves the cylinder and flows into the exhaust. In this case, the sensor voltage goes lower than 0.45 volts. Usually the output range seen is below 0.2 volts to above 0.8 volts.

The O2 sensor does not begin to generate it's full output until it reaches about 600 degrees F. Prior to this time the sensor is non conductive. It is as if the circuit between the sensor and computer is not complete. The mid point is about 0.45 volts. This is neither rich nor lean. A fully warm O2 sensor will not spend any time at 0.45 volts. In many cars, the computer sends out a bias voltage of 0.45 volts through the O2 sensor wire. If the sensor is not warm, or if the circuit is not complete, the computer picks up a steady 0.45 volts.

Since the computer knows this is an "illegal" value, it judges the sensor to not be ready. It remains in open loop operation, and uses all sensors except the O2 to determine fuel delivery. Any time an engine is operated in open loop, it runs somewhat rich and makes more exhaust emissions. This translates into lost power, poor fuel economy and air pollution. The O2 sensor is constantly in a state of transition between high and low voltage. Manufacturers call this crossing of the 0.45 volt mark O2 cross counts. The higher the number of O2 cross counts, the better the sensor and other parts of the computer control system are working. It is important to remember that the O2 sensor is comparing the amount of Oxygen inside and outside the exhaust system.

If the outside of the sensor should become blocked, or contaminated with oil, sound insulation, undercoating or antifreeze, (among other things), this comparison is not possible. This makes the regular checking of the Oxygen sensor output and cycle rate (number of cross counts) of great importance. Keeping the sensor fresh may improve fuel economy as much as 5-15% (which can save hundreds of $$$ each year in fuel costs). Keeping the sensor in good operating condition will also minimize exhaust emissions, reduce the risk of costly damage to the catalytic converter, and insure peak engine performance (smooth operation, no surging or hesitating).

For these reasons, the O2 sensor operation should be inspected during a tune-up to insure that it is operating properly. Sometimes O2 sensor replacement is necessary, especially on older vehicles (those built before the mid-1990's).

Key Benefits

Other services that should also be considered when tuning up a vehicle to achieve maximum results.

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